1. Field of the Invention
This invention relates generally to the field of small engine control and specifically to an air/fuel mixture adjustment method using an improved dilution tunnel for collecting emission gasses.
2. Description of the Related Art
Exhaust emissions from internal combustion engines in automobiles and other vehicles are strictly regulated by the United States Government and other authorities. Emissions from vehicles and machines using smaller engines, such as chainsaws, lawn mowers, trimmers, and blowers are also a matter of concern. Most commonly these are two-stroke engines which are susceptible to scavenging losses and other phenomena which affect emissions and performance. Thus, numerous regulations and standards have been promulgated setting forth acceptable levels of emissions from such engines. Engines also must comply with regulations regarding air/fuel ratios. To measure emissions and determine compliance with the regulations, it is necessary to test the emissions and performance of such engines.
The air/fuel ratio of an engine is an important factor in measuring and controlling performance of the engine. If the mixture is too rich, the fuel does not burn completely, fuel is wasted, and unburned fuel (including HC and CO) is exhausted or fouls the engine. If the mixture is too lean, the engine loses power, knocks, or fails. The optimum mixture varies with the speed and operating conditions of the engine. Setting the air/fuel mixture of small engines is conventionally accomplished by adjusting one or more needle valves that control fuel flow in the carburetor.
As mentioned above, engine emissions are regulated by government and industry standards to reduce pollution and improve fuel economy. Therefore, it is advantageous to measure the air/fuel ratio to ensure compliance with the standards. More importantly, it is desirable to adjust or control the air/fuel ratio to achieve compliance with the standards or to achieve desired performance characteristics.
Numerous gas analyzers are known and available for determining the nature and quantities of the various components comprising the exhaust gas, including carbon monoxide, carbon dioxide, and hydrocarbons. Results are calculated based on measured levels in accordance with SAE J1088 and other standards.
To achieve accurate results it is necessary to properly collect the gas to be analyzed. In many cases, collection and testing are performed during final assembly of the machines or vehicles. Many engines are tested in succession by unskilled workers. Thus, gas collecting and testing apparatus should be simple to operate and easy to move from one engine to the next. One known collection method uses a probe inserted into a muffler of the exhaust system of an engine to be tested. The probe directs a sample of the exhaust to the analyzing apparatus. This method is relatively simple, but is highly dependent on the location of the probe. Different results are achieved at different locations and inaccurate results are obtained if the probe is not properly located. Typical causes of inaccuracies are incomplete mixing of the gasses in the muffler and introduction of atmospheric air into the sample. Inserting probes into the mufflers of engines on an assembly line is burdensome and prone to inconsistencies.
More consistent and accurate results have been achieved with an apparatus known as a mixing chamber, as described in SAE J1088. This is a heated metal box fastened to the outlet pipe of the muffler. The components of the exhaust gas are completely mixed in the chamber before being admitted to the sample probe and directed to the analyzing apparatus. The volume of the mixing chamber is at least 10 times, and usually 100 to 200 times, the cylinder displacement of the engine being tested. However, the size must be selected so that the temperature inside can be maintained at a level which will prevent hydrocarbons from condensing. The outlet of the chamber must be constructed so as not to have a tuning effect on the engine. The design of the chamber and the fastening means may affect engine performance, thus, mixing chambers are uniquely designed for each type of engine tested. Whereas mixing chambers are accurate, they are bulky, complex, and require substantial set-up time. Attaching the mixing chamber to engines on an assembly line is burdensome and time consuming. Maintaining adequate chamber temperature is an additional problem.
A dilution tunnel is another device used to collect the exhaust gasses for analysis. The dilution tunnel has one inlet which is fastened to the exhaust outlet pipe. A second inlet is connected to a source of dilution air or other gas which is to be mixed with the exhaust gas prior to analysis. It is well known in the art that it is desirable to dilute the exhaust gas with atmospheric air in a carefully controlled proportion. Dilution tunnels of this type are shown in U.S. Pat. Nos. 3,699,814; 3,817,100; 3,892,549; 3,965,749; 3,986,386; 4,586,367; 4,633,706; 4,654,058; 4,660,408; 4,747,297; 4,974,455; 5,058,440; 5,090,258; and 5,184,501. As with mixing chambers, the disadvantages of dilution tunnels according to the prior art are bulk, complexity, and long setup time. Also, it is necessary to fasten the tunnel to the exhaust outlet and a source of dilution air, which is troublesome in an assembly line where many engines are tested in succession.
Accordingly, it is desirable to have a device for collecting and analyzing exhaust gasses which is small, lightweight, and easily set up. Such a device should be adaptable for testing large numbers of engines in succession and should be simple and inexpensive to operate, while providing consistent and accurate results. The measured emissions of the engine should be used to determine and adjust the performance of the engine. In particular, the device should be connected to a system which allows measurement and adjustment of the air/fuel ratio in response to results of exhaust gas analysis for the engine.